Semaphore flies

After a wildlife-packed June with the 30 days wild challenge, I felt like I needed a new focus and I decided to carry on a wildlife challenge by blogging/tweeting about an invertebrate I've seen that day for July. Yesterday, while walking by the local park pond I saw a Speckled Wood. The butterfly flew away, but it made me notice the flies dancing on the Colt's Foot leaves: they were Semaphore Flies, Poecilobothrus nobilitatus, a whole swarm of them signalling to each other. Although small, this species is very easy to identify by their smoky wings with a white tip. They are very active in July on muddy puddles and ponds.

Flies dancing on water

Approach a pond in July or August and crouch down. Focus on the surface of the water, checking the floating leaves or debris by the water edge and you will likely see jittery, shiny green flies. Get comfortable and watch them for a while, it takes some time until you can transport yourself to this small, alien world. These are long-legged flies, Poecilobothrus nobilitatus. They like to rest on floating objects or the mud on the edge (above), but they are also able to walk on the water surface. You will notice that some flies, the most active ones, have smoky wings with bright white wing tips - they are the males (top shot). Clusters of males may dance around single females, which lack the white wing marks (below).

Female walking on the water surface

To start courtship, the male focus on a female and approaches her, then he opens his wings in straight angles to his body and buzzes them quickly before closing them again, the wing-waving display, which is just a component of their complex and spectacular courtship. 

 Here several males circle a female wing waving, although the wings are barely visible as they are buzzing them. The three males court a female, which is watching the male in the right.

A male in the middle of wing waving

I took a little clip of their courtship earlier today.

The details of the courtship are a combination of wing waving and aerial acrobatics too fast for the human eye to follow, but they were described in detail by M.F. Land, which examined an hour of videos of courting flies in a pond in Sussex. The following figure from his paper represents the longest bout of filmed courtship, which was interrupted by the male chasing another male. Wing waving upon approach to a female is followed by hovering in front of her and flying over the female and back, and more wing waving. Courtship is often interrupted by chases to other males and then returns to the same female to carry on.

(from Land, 1993)

UPDATE: Thank you to Morgan Jackson for pointing out that this fly was one of the species in the Name a Species 2012 competition and won the lovely English name of Semaphore fly.

More information

Land, M.F. (1993). The visual control of courtship behaviour in the fly Poecilobothrus nobilitatus J Comp Physiol A, 173, 595-503 DOI: 10.1007/BF00197767